This invention relates to an improved metal flow system, for use in the production of pressure castings made from magnesium alloys in a molten or thixotropic state and suitable for use with existing machines in various forms including hot and cold chamber die casting machines.
An understanding has developed throughout the international pressure casting industry that, because of the lower heat capacity of magnesium alloys compared to zinc and aluminium alloys, it is necessary to use large runners and gates to prevent premature freezing of the molten magnesium alloy metal. Indeed, this is considered best practice by the industry, although interpretations vary considerably.
Within the industry, there are many different design methods which are thought to provide satisfactory castings from magnesium alloys. However, the magnesium alloy pressure castings produced by these methods generally exhibit a greater degree of surface defects, when compared to zinc or aluminium pressure castings, although castings may be of servicable quality.
We have found that it is possible to produce high quality pressure castings of magnesium alloys with use of the present invention. The castings so produced are able to be of a quality comparable to that obtainable with castings of aluminium or zinc alloys. Moreover, we have found that casting quality is able to be enhanced by the use of metal flow systems having runners and gates which are small relative to current best practice. The metal flow systems of the invention enable a substantial improvement in the casting yield; that is, in the percentage ratio of casting weight to total shot weights. Thus, the weight of metal which needs to be recycled and reprocessed is able to be substantially reduced, with resultant reduction in production costs.
The present invention enables a method of calculating metal flow systems for the production of magnesium alloy castings which exhibit improved quality and with significantly less metal in the feeding systems, with consequent reduction in cost compared to prior practices.
The present invention provides or uses, for the pressure casting of magnesium alloy in a molten or thixotropic state with a pressure casting machine having a mould or die which defines a die cavity, a metal flow system which includes a die or mould tool means which defines at least one runner from which molten magnesium alloy is able to be injected into the die cavity. In a first form of the invention, the metal flow system is of a form providing for control of metal flow velocities within the flow system, whereby substantially all of the metal flowing throughout the die cavity is in a viscous or semisolid state.
The invention also provides a process for producing a casting of a magnesium alloy, wherein the magnesium alloy is cast in a molten or thixotropic state, using a pressure casting machine having a mould or die which defines a die cavity, and using a metal flow system which includes a die or mould tool means which defines at least one runner of the system from which molten magnesium alloy is injected into the die cavity, and wherein the flow system is of a form whereby it provides for control of metal flow velocities therein whereby substantially all of the metal flowing throughout the die cavity is in a viscous or semi-solid state.
Our findings indicate that, with the attainment of a viscous or semi-solid state, filling of the die cavity proceeds progressively by semi-solid fronts of metal moving away from a gate or other site of injection. This form of filling with magnesium alloy is a major departure from the highly complex liquid peripheral fill, followed by back-filling, encountered with die casting of aluminium or zinc alloys and first described by Frommer in 1932 (see the reference text xe2x80x9cDie Castingxe2x80x9d by H. H. Doehler, published 1991 by McGraw-Hill Publishing, Inc.
In the first form of the invention, the flow of magnesium alloy from the runner is via at least one controlled expansion region of the metal flow system in which region the metal flow is able to spread laterally, with respect to its direction of injection, with a resultant reduction in its flow velocity relative to its velocity in the runner. In a preferred arrangement, the controlled expansion region of the flow system comprises a gate through which the metal flows from the runner to the die cavity. In that preferred arrangement, the gate and runner are such that an effective cross-sectional area of flow through the gate exceeds an effective cross-sectional area of flow through the runner, whereby the molten metal has a velocity through the effective cross-sectional area of flow through the runner which exceeds its velocity through the gate. This is contrary to current recommended practice.
In that preferred arrangement according to the first form of the invention, the cross-sectional area of flow through the gate preferably exceeds the effective cross-sectional area of flow through the runner to an extent providing for a ratio of those areas in the range of about 2:1 to 4:1.
The effective cross-sectional area of flow through the runner may prevail throughout the full longitudinal extent of the runner. However, the effective area may prevail over only part of that longitudinal extent. Thus, in the latter case, there may be a larger cross-sectional area of flow through the runner up-stream from the part of its longitudinal extent in which the effective cross-sectional area of flow prevails.
In an alternative arrangement according to the first form of the invention, the controlled expansion region is defined at least in part by and within the cavity, by surfaces defining the cavity adjacent to the site at which the metal enters the cavity. In this alternative arrangement, there may be an in gate at that site, through which metal flows from the runner to the cavity. In that case, the gate need not define a controlled expansion region due to it having a larger effective cross-section than the runner, and the gate may simply comprise the outlet end of the runner at the cavity. However, the gate may define part of a controlled expansion region of which a further part is defined by and within the die cavity.
The alternative arrangement, in which the metal flow system has a controlled expansion region, defined at least by and within the die cavity, is not suitable for all die cavity shapes. Also, attainment of such region is dependent upon the flow direction as the metal enters the cavity relative to adjacent surfaces of the cavity. In general, the surfaces need to alloy expansion while controlling it, so as to function in the cavity in a manner similar to a gate providing controlled expansion. As such, a controlled expansion region defined by the cavity can be regarded as a pseudo gate and, in general, a reference in the following to a gate is to be understood as covering both an actual gate and such pseudo gate. However, the die cavity surfaces which define a pseudo gate, through which metal flows on entering the cavity, usually will not contain the flow on all sides, although substantial containment such as on three sides is preferred.
A controlled expansion region may be achieved by a sharp, step-wise increase in cross-section from the effective cross-section of the runner. However, it is preferred that the controlled expansion region progressively increases in cross-section in the direction of metal flow therethrough. Thus, where the expansion region is defined by an actual gate, the gate preferably increases in cross-section to a maximum cross-section where the gate communicates with the die cavity.
The invention is applicable to either hot-chamber or cold-chamber die casting. In each case, the invention enables very substantial cost savings in the production of castings of magnesium, as illustrated later herein, as it enables a substantial improvement in the casting yield. Hence the weight of runner/sprue metal which needs to be recycled and re-processed is substantially reduced, a matter of particular relevance in the casting of magnesium due to the care needed in re-processing.
The metal flow system provided by the invention, and used in a casting process according to the invention, usually is substantially provided by a die or mould part or tool which defines part of the die cavity. However, as with conventional pressure cavity moulds and dies, it may be defined by co-operating parts or tools.
The system of the invention may be adapted for use in pressure casting with a given machine. At least where this is the case in the system and process of the invention, the velocity of molten metal through the runner is preferably about 150 m/s. Variation in this velocity is possible, such as within the range of about 140 to 165 m/s. However, the velocity need not prevail through the full length of the runner, although this is preferred in at least some forms of the invention. Rather, it is sufficient if the velocity is attained over part of the length of the runner which has a lesser effective cross-section than exists over other parts of the length.
The velocity of the flow of molten metal through the controlled expansion region may be about 25 to 50% less than the flow through the runner. In many instances, it is found that the metal velocity through the expansion region is very close to two-thirds of that in the runner. Thus, with a runner velocity of about 150 m/s, the expansion region velocity preferably is about 100 m/s.
In the foregoing, there is reference to an effective cross-sectional area of flow through the expansion region and through the runner, as distinct from the physical cross-sectional area of the expansion region and runner. This distinction is important, as reflected by the initial experiments of the first series of experiments outlined later herein. Those initial experiments were conducted with large runners and gates, in accordance with the prior art best practice for casting magnesium alloys and similar to practice for casting aluminium and zinc alloys. The actual flow path in the runners in those initial experiments was through a cylindrical region much smaller in cross-sectional area than the designed physical cross-sectional area of the runners. The much smaller area of the flow region comprised a somewhat centralised core in which the molten metal flowed through the runners, and which was within a sleeve of at least partially solidified metal of substantial wall thickness. For a given runner cross-sectional area. the cross-sectional area of the flow region was larger when the die was hot.
The relevance of the distinction drawn between an effective flow cross-sectional area through a runner, and the actual or designed cross-sectional area, is less pronounced in a runner of the metal flow system of the invention than in the prior art best practice. Indeed, in a limiting situation according to the invention, the distinction can be substantially eliminated. That is, in the limiting situation, the runner can have a relatively small designed cross-sectional area which substantially defines the effective cross-sectional area of flow through the runner. To facilitate attainment of this situation, an upstream part of the length of the runner of a hot-chamber system may be defined by a member formed of a suitable ceramic material which enables maintenance of temperature cycle inhibiting the solidification of metal on surfaces of the member which define the runner. Alternatively, such upstream part of the length of the runner of a hot-chamber, or for a cold-chamber, system may be defined by a member adapted for the circulation of a heat exchange fluid, or by the use of an electric heating device, to enable maintenance of such temperature cycle.
The prior practices have necessitated large runner systems which, in general, have runners of larger cross-section than their gate, that is, the converse of that enabled by the invention with respect to the cross-sections of the runner and controlled expansion region. As a consequence, they have resulted in a relatively large quantity of runner/sprue metal for a given casting and, hence, high costs in recycling and reprocessing the runner/sprue metal. The prior practices generally have resulted in runner/sprue metal in excess of 50% of the weight of the casting and over 100% in some instances. That is, the quantity of runner/sprue metal can be greater than that of the casting.
In contrast to the prior art practices, the present invention enables the quantity of runner/sprue metal to be substantially reduced, such as to less than 30% of the casting weight for cold-chamber machines. In many instances, particularly with hot-chamber machines, the invention enables the quantity of runner/sprue metal to be well below this level, for example as low as about 5% or even as low as about 2%. This, of course, provides a significant practical benefit, since the cost of re-processing recycled metal is correspondingly reduced.
The present invention enables the quantity of runner/sprue metal to be substantially reduced as a direct result of reduction in the designed cross-section of the runner, with a further reduction being possible by reduction in runner length. The designed cross-section can be reduced so that it substantially corresponds to the effective cross-section of flow through the runner. However, the effective cross-section of flow need prevail along only part of the length of the runner, such as along a minor part of the length. Also, the part of the length of the runner which is solidified in a casting operation is able to be shortened substantially, to achieve a further reduction in the quantity of runner/sprue metal.
The present invention enables the attainment of important benefits beyond that of reducing re-processing costs. These include a significant improvement in the related parameters of casting porosity and surface finish. Relative to die castings of aluminium or zinc alloys, castings of magnesium produced by prior art practices usually have an inferior surface finish, frequently attributable to porosity at or near the casting surface. However, the present invention enables casting porosity to be substantially reduced and also enables the attainment of a uniform surface finish of good quality.
A common factor in reducing the quantity of runner/sprue metal, reducing porosity and improving surface finish is believed to be the attainment of the molten metal flow velocities enabled by the invention. With such velocities, it is believed that, apart from a region of the die cavity adjacent to the controlled expansion region, metal flow in the die cavity is due to the molten metal being in a viscous state. Thus the flow in the die is as of a semi-solid front fill with the percentage solids in the flowing metal remaining relatively constant during filling of the cavity. That is, filling of the cavity appears to proceed by semi-solid fronts moving away from the controlled expansion region, in contrast to the highly complex peripheral fill and back-filling encountered with casting of aluminium or zinc alloys.
The invention as detailed herein is based on a range of experiments. A first series of the experiments were aimed at providing a better understanding of the mechanism of flow and solidification of magnesium alloys. Specifically the experiments sought to establish whether improvements to surface finish and porosity levels could be achieved by changing and/or controlling the physical parameters for specific castings Some of the initial experiments of that first series used the xe2x80x9cshort shotxe2x80x9d technique to gain understanding of the flow patterns. These experiments resulted in the identification of two flow regimes within the cavity which always produced an area of poor finish between them. The flow pattern was unlike any seen in zinc or aluminium pressure castings. Examination of the microstructure showed that:
the flow in the runner was through a cylindrical region much smaller in cross-section than the designed physical runner cross-section. This was also noted in sections of the casting in which the flow was unidirectional.
the percentage solids in the magnesium alloy castings (as demonstrated by dendrites with large dendrite arm spacing) was approximately 50%.
the microstructure of the magnesium alloy castings near the gate was different from that observed from 50 mm to 300 mm from the gate.
The results of these initial experiments seem to suggest that the metal had partially solidified in the runner and then behaved as a semi-solid within the cavity, with attendant viscous behaviour. The first metal travelling along the runner (the front) appeared to have entered the cavity in a liquid state and hence this could explain the different microstructures obtained and the substantially common position across the casting of the transition between these different flow conditions.
In later experiments of the first series, changes to the style of runners and gating within the traditional gating philosophy resulted in marginally improved castings, whereas large changes were expected in accordance with that philosophy. However, the area and position of poor surface finish remained substantially unchanged. A radical change to a single taper tangential runner produced an extremely good result when considering the quality of the casting, but the product to runner/sprue ratio was not acceptable. The general level of understanding of the flow behaviour at this stage was extremely limited. However, what was apparent is that magnesium alloys behave significantly differently to zinc and aluminium alloys.
A second series of experiments was carried out with a number of different dies and casting machines to try to establish if the difference in behaviour was due to thixotropy.
The experiments covered various casting sizes ranging from 15 grams to 15 kg and were carried out on both hot and cold chamber machines. In one of the experiment with a very long casting (approximately 2 m) which comprised a series of open ended boxes, the casting was fed along the long edge in a cold chamber machine. Two large runners from the sprue fed long semi-tapered runners. It was our contention that if the metal was in a thixotropic state in the cavity then it should be possible, due to viscous heating, to fill the casting from one end. To prove this, a section of a previously cast runner was replaced in the die, thus effectively blocking off the metal entry to that half of the cavity. Therefore any metal in the cavity adjacent to the blocked off runner must have entered from the unblocked side, producing flow distances in excess of 1 meter. The flow path in the cavity was extremely complex and exhibited many changes in direction. However, with no change in maching settings, the one sided feeding system produced a casting, the quality of which was superior at its extremes to those produced with complete runners. The significant change noted was an increase in metal velocity.
Additional experiments of a third series were conducted with a casting 280xc3x9725xc3x971 mm made in a small hot chamber machine and fed with a long thin runner and extremely thin gates of 0.15 mm deep. These experiments showed that the gate was badly blocked along much of its length resulting in poor quality castings. The runner, which was 220 mm tong in one direction, was reduced to an effective length of 100 mm by welding a plug 10 mm long into the runner. The resultant casting was totally filled and metal flowed from the cavity into the unblocked portion of the runner through the 0.15 gate. This demonstrated that the alloy was in an extremely low viscosity state throughout cavity fill. Similar castings in zinc or aluminium alloys would not exhibit this characteristic. It should be noted that the machine exerted a pressure of only 14 MPa on the metal.
Examination of magnesium castings produced by the best practice use of long thin gates invariably show that large sections of the gate in fact are not working.
Further experiments of a fourth series were carried out in a range of castings sizes, but all exhibited that the quality improves when gates and runners are reduced in size and metal velocity increases. Examination of runner cross-sections, ranging from 1xc3x971 mm to 50xc3x9750 mm, from a number of castings produced on both hot and cold chamber machines, revealed in each case a central circular region. This characteristic did not appear to be influenced by the original cross-sectional profile. The presumption for this condition is that it defines the region where metal flow occurs during cavity fill and is assumed to be the effective flow cross-section. Because this region is smaller in cross-sectional area than the runner channel as originally cut in the die, metal flow achieves a significantly higher velocity. Calculations, using measured metal flow rates, result in values for runner velocities which cluster around 150 m/sec, with gate velocities being approximately 2/3 that of the runner velocity. Similar regions can be found in castings where there is unidirectional flow.
A fifth series of experiments involved producing a long thick casting through progressively smaller gate sections. The original gated length was reduced from 120 mm to 8 mm and the castings remained of acceptable quality. Micro examination of the castings showed that the filling was consistent with a semi-solid front fill, and the percentage solids during fill remaining constant throughout the part. Porosity was minimal.